When non-inbred rats are given freedom of dietary choice, the quantity and composition of the diet selected varies from animal to animal in a way that greatly increases the risk to the individual of developing a neoplasm. Two types of long-term experiments are underway to explain the maximization of risk under these self-selection conditions. One series of studies are designed to test the hypothesis that there is individual specificity in the dietary conditions that modulate the susceptibility to certain tumors and also to determine whether this individual specificity is dependent on genetic diversity. Another series of studies are designed to test the validity of an empirically derived multi-factorial model that defines both the dietary and growth-related conditions associated with the development of tumors and those conditions that minimize tumor risk.